The present invention relates to an exhaust gas cleaning apparatus for an internal combustion engine.
In a known apparatus for cleaning exhaust gas for an internal combustion engine mounted on a vehicle, a particulate filter for collecting particulate matter contained in exhaust gas is provided in the exhaust passage of the internal combustion engine, and a selective NOx reducing catalyst is provided downstream of the particulate filter in the exhaust passage so as to remove NOx from the exhaust gas. In such cleaning apparatus, a dosing valve is disposed downstream of the particulate filter and upstream of the NOx reducing catalyst in the exhaust passage. Urea solution is injected into the exhaust passage by the dosing valve so that the NOx reducing catalyst reduces NOx in the exhaust gas with the injected urea solution to thereby clean the exhaust gas.
Japanese Unexamined Patent Application Publication No. 2008-63968 discloses an apparatus for cleaning exhaust gas for an internal combustion engine that regenerates or cleans the particulate filter. In the cleaning apparatus, temperature of the particulate filter is raised at specified timings to thereby clean the particulate filter by eliminating the particulate matter collected in the particulate filter. As a result, the amount of the particulate matter collected in the particulate filter is prevented from increasing excessively. Such filter regeneration process is performed not only when the vehicle is traveling at a high speed and hence the temperature of the exhaust gas is high, but also when the vehicle is traveling at a low speed on a congested road. The filter regeneration process is also performed during an idle operation of the internal combustion engine, that is, when the amount of fuel combusted is small and therefore the temperature of the exhaust gas is decreased.
In the above exhaust gas cleaning apparatus, some part of the urea solution injected from the dosing valve adheres in and around the injection hole of the dosing valve. When the temperature of the dosing valve and its vicinity rises, for example, to a temperature T1 or higher, urea in the solution adhered in and around the injection hole of the dosing valve precipitates and the precipitated urea deposits there.
When the temperature of the dosing valve and its vicinity rises further to a temperature T2 (where, T2>T1), the urea that has deposited in and around the injection hole decomposes and disappears. The particulate filter is heated in the course of the filter regeneration process and the exhaust gas that has passed through the heated particulate filter heats the injection hole of the dosing valve and its vicinity thereof to the temperature T2, with the result that the urea deposits in and around the injection hole disappears by decomposition.
In decomposing and eliminating the urea that is present in and around the injection hole of the dosing valve by means of the heat of the exhaust gas that is heated in the filter regeneration process, if the filter regeneration process that is performed during an idle operation of the internal combustion engine is repeated, the urea that is present in and around the injection hole may not only be decomposed, but the deposition of the urea may rather be accelerated.
Specifically, when the internal combustion engine is running idle, the temperature of the exhaust gas is low. Therefore, in the filter regeneration process that is performed during an idle operation of the internal combustion engine, the exhaust gas may not be able to heat the injection hole of the dosing valve and its vicinity to the temperature T2 or higher. When the temperature of the injection hole of the dosing valve and its vicinity is the temperature T1 or higher but not higher than the temperature T2, the urea precipitates. The temperature that is equal to or higher than the temperature T1 but lower than the temperature T2 is the value at which the urea precipitates and therefore, deposition of the urea in and around the injection hole of the dosing valve is accelerated if the filter regeneration process is performed repeatedly. Accordingly, the repeated filter regeneration process during an idle operation of the internal combustion engine may accelerate the deposition of the urea in and around the injection hole of the dosing valve.
If deposition of urea occurs progressively, the injection hole of the dosing valve may be blocked and the dosing valve may not be able to inject urea solution properly, with the result that a failure may occur in the reduction of NOx by use of the NOx reducing catalyst.
In view of the above circumstances, the present invention is directed to providing an exhaust gas cleaning apparatus for an internal combustion engine that prevents urea from being deposited in and around the injection hole of the dosing valve with the filter regeneration process.